Britain's AAA credit rating has been put on a "negative outlook" by ratings agency Moody's amid fears over weaker growth prospects and potential shocks from the eurozone crisis.

Chancellor George Osborne said the assessment was a vindication of the Government's tough austerity measures and "a reality check for anyone who thinks Britain can duck confronting its debts".

Moody's downgraded the ratings of six countries and also put France and Austria on the same caution as the UK amid violent protests in Greece over stringent measures to secure a fresh bailout.

Explaining its decision on the UK's prospects, it pointed to "increased uncertainty regarding the pace of fiscal consolidation in the UK due to materially weaker growth prospects over the next few years".

"Any further abrupt economic or fiscal deterioration would put into question the Government's ability to place the debt burden on a downward trajectory by fiscal year 2015-16," it said.

It also predicted that the "high risk of further shocks (economic, financial, or political) within the currency union are exerting negative pressure".

Mr Osborne said: "This is proof that, in the current global situation, Britain cannot waver from dealing with its debts. Moody's are explicit that it is only the Government's 'necessary fiscal consolidation' that is stopping an immediate downgrade, which would happen if there were any 'reduced political commitment to fiscal consolidation including discretionary loosening'.

"This is a reality check for anyone who thinks Britain can duck confronting its debts."